This invention relates to a web guidance mechanism particularly for the guidance of a web of filter cloth material in a continuous belt filter.
In known continuous filter equipment the filter medium is usually an endless belt made of a porous cloth material arranged around end pulleys as in a belt filter or partially around a suction drum as in a rotary vacuum belt filter. These belts of filter material have to be guided or tracked to ensure that they run centrally of the equipment and do not foul or otherwise inhibit the correct operation of the machinery. Such tracking is also essential to ensure that the cloth material itself is not damaged during use.
A common known method of providing such tracking is to use two sets of nip rollers which are disposed at an angle of approximately 10.degree. to the travel of the belt. The nip rollers may be normally open or engaged and cloth sensors are provided on each side of the cloth so that movement of an edge of the cloth with respect to its normal path may be detected. In the case of normally open rollers a sensor would be activated by the edge of the cloth moving towards it causing the nip rollers to be closed on the opposite side of the material thus pulling the web back with respect to the activated sensor and thus ensuring normal tracking. In a system where the nip rollers would be normally closed activation of one or more sensors would open the rollers thus allowing the cloth web to move in a reverse manner to that previously described. In most of these systems the sensor and the nip rollers are mounted in one common unit. These web tracking or guidance mechanisms have been for the most part, developed from the textile industry where similar tracking and guidance control is required during rolling or other manufacturing operations. Such systems are optimised for a situation where the cloth web is only tracked once and hence any slight amount of damage done to the edge of the cloth by the nip rollers is of little consequence. In a continuous belt filter the filter cloth may run for many months before being renewed and consequently would have to pass many times, perhaps hundreds of thousands, through the nip rollers or other guidance mechanisms. Any undue strain applied to the filter cloth by these mechanisms can therefore seriously weaken the cloth structure and lead to damage and/or uncertain filtration characteristics.
Furthermore, most known filter cloths tend to stretch during usage to the extent that the width of the cloth narrows slightly. In this instance the sensors would have to be repositioned at frequent intervals to allow for this. If such repositioning is not performed excessive movement of the cloth results which would require an excessive force on the part of the mechanism to ensure that the cloth is pulled back to its correct track. If too much displacement of the web occurs the tracking mechanism may not be able to pull the cloth back to its central position Furthermore, if the sensor and nip rollers are in one unit the repositioning of the sensors also requires repositioning of the nip rollers with the possibility that the nip angle (usually 10.degree. as mentioned previously) would be altered during the positioning process. One further problem associated with the peculiar characteristics of filter cloths is the tendency to form wrinkles or folds. In order to prevent this it is advisable to ensure that the cloth is at all times under a slight tension especially prior to passing under the turning roller or rollers.